Thermistor control circuit



Jan. 13, 1953 E. LAKATOS ET AL THERMISTOR CONTROL CIRCUIT 5 Sheets-Sheet l Filed Nov. 25, 1947 -.LKATOS NVENTORSBlMcM/LLAN ATTO/gy Jan. 13, 1953 E. LAKATOS ET AL 2,625,606

THERMISTCR CONTROL CIRCUIT Filed Nov. 25, 1947 s sheets-sheet 2 /NVENTORS-ELAKATOS B. MFM/LAN ATTORNEY Patented Jan. 13, 1953 THERMISTOR CONTROL CIRCUIT Emory Lakatos, Cranford, and Brockway McMillan, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a

corporation of New York Application November 25, 1947, Serial N o. 787,984

(Cl. {Z8-44) 23 Claims.

This invention relates to electrical circuits and methods employing a plurality of current-controlled Variable impedance elements and it has for its principal object the maintenance of a predetermined constant relation between the impedance values of the elements throughout a range of values of control currents.

A more particular object is to maintain the resistance values of a pair of such impedance elements exactly equal to each other throughout the range of Variation of control currents despite dissimilarity of the resistance versus control-current characteristics of the two elements.

A further object is to maintain exactly equal the respective gains of a pair of amplifiers that are simultaneously varied in gain by a common control current operative on respective gain-controlling variable resistors that have slightly dissimilar characteristics.

Still another object is to reduce the inaccuracy present in certain types of voltage multiplying circuits due to dissimilarity of the characteristics of the current-controlled variable impedance elements employed therein.

One type of current-controlled device to which the invention is particularly applicable is the thermistor. The term thermistor designates a circuit element whose electrical resistance varies rapidly with changes in temperature. In contrast with metals, which have small positive temperature coeiiicients of resistance, thermistors are usually made from a class of materials, known as semi-conductors, which have relatively large negative coeicients. According to the present invention, voltage from an alternating current source, preferaby at a frequency considerably removed from the operating frequency of the system oi which the thermistors are a part, is applied at a comparatively ,low level to each of the two thermistors and the resulting currents are compared by electrical means. Variations of the two currents from a predetermined relation are detected and employed to modify the resistance of one or both of the thermistors to regain the original relationship.

A more thorough understanding of the inven tion may be obtained by a study of the following ldetailed description of several specific embodiments. These embodiments are shown in the drawings, in which:

Fig, 1 represents one embodiment of the invention which is capable ofrelatively general applica tion and illustrates some' of the principlesinvolved';

I2 snows a wave transmission system employing two similar amplifiers and means in accordance with the invention for regulating the gains of those amplifiers equally;

Fig. 3 shows an alternative circuit in accordance with the invention for regulating equally the gains of the amplifiers of Fig. 2; and

Fig. 4, illustrates a voltage multiplying circuit and means in accordance with the present invention for obtaining a high degree of accuracy in its operation.

In Fig. l, two substantially identical external circuits I and 2 are operating on alternating current which is, for all practical purposes, conned to those circuits. Two self-heated thermistors 3 and 4 are connected, respectivelyto each of the external circuits I and 2 in such a manner as to enable a change in the resistance of a thermistor to change some property of the external circuit with which it is associated. For example, each thermistor might control the gain of an amplifier circuit, as in Fig. 2.

An alternating current source 5 of control voltage operates at a frequency quite small with respect to the operating frequency of the external circuits I and 2. The primary winding 6 of a transformer 'I is connected across the terminals of the control source 5. One side of the control source 5 is grounded. An alternating current source 8 of calibration voltage operates at a frequency intermediate between the frequency of the control source 5 and the operating frequency of the external circuits I and 2. The primary winding 9 of a second transformer I0 is connected across the terminals of the calibration source 8. One side of the calibration source 3 is also grounded. The secondary winding I I of the first transformer 'I is connected in series with the secondary winding I2 of the second transformer I5. rlC-he remaining terminal of the secondary winding l2 of the second transformer I0 is grounded.

The remainder ofthe circuit is as follows: The free end of the secondary winding II of the first transformer 'I is connected to one side of one of the thermistors 3. This same side of the thermistor 3 is connected through a resistance I3 to one input terminal of a high gain amplifier I4. The other amplifier input terminal is grounded. One output terminal of the amplifier I4 is also grounded and the other output terminal is connected to the other thermistor 4. A shunting resistor I5, connected between the ungrounded input .and output..terminals of the amplifier I4, furnishes anegative feedback circuit. The other sides of the thermistors 3 and 4 are connected to respective input terminals of a band-pass filter I6, designed to pass only the calibration frequency. A third input terminal of the band-pass lter It is grounded. One output terminal of the band-pass filter I6 is grounded and the other is connected to a modulator and phase detector Il. A nal connection goes from the modulator and phase detector through a resistor I8 to a point between the resistor I3 and the ungrounded input terminal of the amplifier I4.

The modulator and phase detector I1 comprises two transformers I9 and 2D, two multigrid tubes 2| and 22, and a band-pass filter 23. The primary winding 24 of one transformer I3 is connected between the ungrounded output terminal of the band-pass filter I6 and ground. The secondary of the transformer I9 is centertapped and the portions 25 and 26 thus formed are connected to the control grids of tubes 2| and 22, respectively. The midpoint is connected to the ungrounded side of the calibrating voltage source 8. The cathodes of the tubes 2| and 22 are connected together and aregrounded through a common biasing resistor 2l. The primary 23 of the other transformer 20 is connected between ground and the ungrounded side of the control voltage source 5. The secondary of this transformer 20 is also center-tapped and the portions 29 and 36 thus formed are connected to the screen grids of tubes 2| and 22, respectively. The mid-point is connected to the positive terminal of a battery 3|. The negative terminal of the battery 3| is grounded. The plates of tubes 2| and 22 are connected Vto input terminals `of the band-pass filter 23, and a third input terminal is connected to the positive side of a battery IB?, the negative side of which is grounded. One output terminal of the band-passv filter 2.3, the filter being designed .to pass only the control frequency, Vis grounded and the other is connected to the resistance I8.

The approximate resistance of each of the thermistors 3 and 4 is under the control of the control voltage source 5. The first thermistor 3 is fed directly from the secondary II of the control voltage transformer |I since the low input impedance of the band-pass filter I6 effectively shorts the low frequency control current to ground. Current also ows from the same secondary winding II through the resistance I3 to the input of the shunt feedback amplifier I4. This input is very nearly at ground potential due to feedback action and, for all practical purposes, the control current flows to ground at this point.

Negative feedback action of the amplier I4 gives an output voltage equal to that appearing across the resistor I3. Therefore, a voltage is impressed on the second thermistor 4 which is equal to that impressed on the first thermistor 3 by the control voltage 'source 5. The Varnplier I4 also contains some means, such as a phase reversing transformer, for reversing the phase of the impressed voltage. Thus, the currents through the thermistors 3 and 4 are opposite in phase. The circuit is completed at the input side of the band-pass filter I6, since the control current is effectively shorted to ground. Changes in the control signal amplitude cause changes in the currents of that vfrequency nowing in the thermistors 3 and 4 and thus control their resistances.

Voltage from the calibration source 8 is iinpressed on the thermi'stors 3' and4. in much'the same manner as is. the. control voltage. Insofar as. the calibration source 8k is concerned, all circuit componentsplay substantially the* samerole as before, with the exception of the band-pass iilter I6 which transmits calibration frequency current. There is no short to ground for this current. As with the control voltage, the calibration voltage sets up currents in the thermistors 3 and 4. The phase reversing feature of the ampliiier I4 causes these two currents to be opposite in phase. The two currents are summed in the input of the band-pass lter I6. The amplitude of the output is proportional to the difference between the amplitudes of the two currents because of the phase reversal of the current through the second thermistor 4. The phase of the output depends on which current is the larger. The modulator and phase detector I? is ysensitive to both amplitude and phase of the diiference current and modulates the signal from .the control voltage source so that the current output of its band-pass filter 23, or error signal, has both the amplitude and phase of the difierence current and has. the frequency of the control signal. Finally, this error signal is introduced through the resistance I3 to the ungrounded input terminal of the amplifier I4. As a result, a voltage equal to the sum or difference, depending on the direction of error, of the control voltage and the error voltage developed across the resistance vI 8 acts on the second thermistor 4. This forces the second thermistor 4 to adjust itself until the error current vanishes; that is, .until the. resistances. of the two thermistors 3 and 4 become equal.

Another embodiment of the invention is. shown in Fig..2. SinceY thesystem is substantially symmetrical, like designations will be used to denote corresponding circuit elements in the following description4 and inthe drawing.

The present embodiment comprises a signal transmission system which contains two similar amplifiers 33 in .parallelwith each other. Either of these amplifiers 33 is able to take over the full load of the system in case of failure of the other. The general level of the gains of the two amplifiers 33 is determined by a `pilot voltage which is transmitted over the line along with the signal.

A signal .transmission line is connected to the input. terminals of 'ar coupling network 32. One output terminal of the network 32 is grounded and the other two terminals, balanced to ground., arev connected, respectively, through similar ampliers 33 to 'the balanced input terminals of a second .coupling network 34. A third input terminal of the second'network 34. is grounded. The output terminals of the second network 34 are connected to a continuation 'of' the signal transmission line.

Each amplifier 33 is a three-stage negative feedback amplifier with a coupling impedance Z in the input .circuit ofthe first stage, interstage networks'NW between the first and second and the second and third stages, and a coupling impedance Z2 .in the output circuit of the third stage. .The incoming Vsignal is fed into the amplifier between the control grid of the first stage and ground, while the output is taken olf from between the plate of the final stage and ground.

Each amplifier 33 has a feedback circuit comprising an impedance Z3 connected between the cathode leads of the first and third stages, a couplingvimpedance Z6 connected between the cathode lead ofthe third'stageand ground, and

la 'coupling circuit including an impedance Z4 .inseries with another impedance A.Z5 connected between the cathode lead of the first stage and ground. Impedance vZ5 is shunted by a condenser 35 in series with a self-heated thermistor 35. Due to negative feedback, the gain of each amplifier 33 is dependent upon the resistance of the thermistor 36 in its feedback circuit.

The general level of the gain of each amplifier 33 is under the control of amplitude variations of a pilot voltage which is transmitted along the line at a frequency which is different from the main signal frequency of the line. This pilot signal is recovered from the output coupling network 34. A pilot filter 31 is connected across the mixed outputs of the amplifiers 33 and is de signed to pass only current of the pilot frequency. The output of the pilot filter 3'! is fed into a rectier 38. The output of the rectifier 38 is then available for use in regulating the gains of the amplifiers 33.

In each half of the circuit, a battery 39 supplies direct current to the thermistor 36 through a multigrid tube 40. The direct current so supplied is the principal heating current for the thermistor 36. The negative terminal of the battery 33 is connected to ground, while the positive terminal is connected to the plate of the tube 46. The cathode of the tube 46 is then connected to a series combination of two inductances 4l and 42 which are, in turn, connected to a point between the condenser 35 and the thermistor 35. Since the condenser 35 blocks direct current, the main heating current flows through the thermistor 36 to ground, completing the circuit. This main heating current is varied by applying rectified pilot voltage from the rectifier 38 to the control grid of the tube 40. The tube acts as a valve and variations of the amplitude of the rectified pilot voltage result in changes in the amount of direct current flowing through the thermistor 36. These current changes affect the resistance of the thermistor 36 and thus determine the general level of the gain of the amplifier 33.

In the present embodiment of the invention, the amplifier circuits 33 are designed to operate at frequencies in the neighborhood of one hundred kilocycles and over. Hereafter, such frequencies will be referred to as radio frequencies. Similarly, frequencies in the neighborhood of one kilocycle, which will be referred to later, will be called audio frequencies, and zero frequency current and direct current will be considered synonymous. The use of these frequency designations is merely for the sake of brevity and clarity and is not intended to limit the application of the invention to the stated frequency ranges.

If the two thermistors 36 had identical re sistances for equal heating currents, over a broad range of heating current amplitudes, the two amplifiers 33 would have identical gains over that range. To obviate the'need for thermistors having such identical resistance-versus heating current characteristics, which it might be difficult if not practically impossible to obtain, this ernbodiment of the invention comprises means to equalize the resistances of the two thermistors 35 for substantially all pilot signal amplitudes and thus insure identical amplifier gains over that range.

An audio frequency voltage source 43 is con nected to the primary winding 44 of a transformer 45. The secondary winding of the transformer' 45 is center-tapped and one end of the primary winding 43 of asecond transformer 4l is attached at the mid-point. The other end of that second primary Winding 46 is grounded. The two portions 48 of the secondary winding of the rst transformer 45 form two arms of a bridge circuit. In each half of the circuit, the free end 6 of the secondary Winding portion 48 is connected through several circuit elements to the ungrounded side of the thermistor 36. These circuit elements consist of a series combination of a condenser 49 which has a low impedance at audio frequencies and the inductance 42. The other inductance 4l, which is connected between the first inductance 42 and the cathode of the tube 40 has a high impedance at audio frequencies and serves as an audio frequency choke, forcing substantially all the audio frequency current to pass through the first inductance 42 to the thermistor 36. The first inductance 42 presents a high impedance at radio frequencies and a low impedance at audio frequencies, acting as a radio frequency choke and thus limiting radio frequency current to the amplifier 33 and its feedback circuit. The condenser presents a low impedance at radio frequencies, allowing relatively free passage of radio frequency current, and a high impedance at audio frequencies. The condenser 35 thus serves to block substantially all audio frequency and direct current from any portion of the feedback circuit but the thermistor 36. The path for audio frequency current, then, is through the thermistor 36 to ground, completing the bridge. A third condenser 53, offering low impedance at audio frequencies, is connected between the cathode of the tube and ground.

Since the system is symmetrical, all correspending circuit elements are substantially equal. Therefore, if the resistances of the two thermistors 36 are equal, the bridge formed by the two portions 48 of the secondary winding of the transformer and the two thermistors 36 will be balanced. If the resistances of the two thermistors 36 are not equal, the bridge will not be balanced and an audio frequency voltage will appear across the primary winding 46 of the second transformer 4l. The phase of this unbalance voltage will depend upon the direction of the unbalance in the bridge. r

A phase sensitive detector is used to convert bridge unbalance into a properly phased direct current control signal. This signal takes the form of a differential voltage and is applied to the screen grids of the tubes 40. The differential effect stems from the fact that one grid is raised in potential while the opposite one is reduced. Tw@ triodes 5l form the basis of the phase sensitive detector. The secondary Winding of the second transformer 41 is center-tapped, forming two portions 52. In each half of the circuit, the end of this portion 52 is connected to the grid of the triode 5l. A potentiometer 53 is connected with its resistance element in parallel with the audio frequency source 43. The resistance element is grounded at an intermediate point and the movable contact is connected to the .center point of the secondary winding of the second transformer 41. In each half of the circuit, the plate of the triode 5I is connected to the screen grid of the multigrid tube 40. The cathodes of the two triodes 5l are connected through a common biasing resistor |66 to ground. A pair of eries condensers 54 presenting low impedances at audio frequencies is connected between the plates of the tivo triodes 5I. Each condenser 54 is shunted by a resistance 55 in order to form a path for direct current. A battery 56 isconnected from a point between the two condenser-sl 54 to ground, the positive 'terminal being connected to the condensers and the negativejter-l mina-1 to ground. fi

The unbalance voltage appearing in the primary windingv 46 of the,A second transformer 47 .also appears,v in the secondary. This voltage alone would swing the grid of one triode positive while swinging the other negative. The voltage appearing on the movable contact of the potentiometer 5-3 swings'the two grids equally and in phase. It is desirable that the amplitud-e of this voltage appearing on the movable contact be greater than the greatest value of unbalanced voltage that is likely to appear in the secondary winding of the second transformer Cil. As a result, any unbalance voltage present adds to the positive and negative excursions of one grid and detracts from those of the other. By proper selection of the magnitude of the biasing resistor |06, the two triodes are biased to conduct `only on positive grid swings, giving an output wave with a directcurre-nt component. The resistance and capacitance. 54 networks act as low-pass filters, the condensers 54 by-passing nearly all audio frequency and above components of the output wave and essentially direct voltages appearing across the resistances 55. rIhus, under control ofthe unbalance voltage, a diierential, direct voltage is applied to the screen grids of the tubes-d. The potential difference between screen grid and ground is raised for one tube while it is being lowered for the other. The opposite effect is. achieved through a. bridge unbalance in the other direction. The valve action of the multigrid tubes 40 enables variations in screen grid potential to control the flow of the main thermistor Siiv heating current. As a result, the resistance of one thermistor 35 is reduced while that ofthe other is raised until the two resistances are equal.

A further embodiment, of the invention is shown in Fig. 3. The circuitv shown is an alternative arrangement for detecting and eliminating thermistor unbalance'in the system of Fig. 2. The output connections of Fig. 3 apply to 1Eig. 2 along the line A--A. Elements common to both drawings `are given like numerical designations.

As in the previous example, the two amplier circuits are controlled by two self-.heated thermistors 36 which yare incorporated in their respective feedback paths. A condenser 35 which passes radio. frequencies but blocks audio frequencies and direct current is, as before, connected in series with each thermistor 35.

The main heating path for one of the therinistors 3SV consists ofA a series combination of a battery 3Q, a` iirst multigrid tube di), the secondary winding 58 of a transformer 59, a radio frequency choke 42, and the thermistor 35.

The negative-side of the battery 39 is grounded and the positive side is connected to the plate of the first multigrid tube 45. The cathode of the rstnmultigri'dltube. 4.0. is connected to the secondary winding 5B of the transformer 5t. Since the side of the thermistor 35. away from the radio frequency chokey i2 is grounded, the circuit is complete. A condenser 5D furnishes an audio irequency path between the cathode of the iirst tube All and ground.

The heating path for the other thermistor 35 consi-sts of a series combination of a battery 39, a second multigrid tube 40, the secondary winding of a second transformer 6i, a radio frequency choke 42, and the thermistor 36. The negative Siae of me battery es, is grounded and the positive side is connected to. the plate ci the second multigrid tube 4B. 111e cathode of the second tube 40 is connected to the secondary winding Gt of the transformer 6l. Again, the side of the thermistor 35 away from the radiofrequency chokey 42 is grounded completing 'the circuit. Here too,` a condenser 5E furnishes an audio frequency path between the cathode of the tube 4i] and rground.

Two substantially equal series resistances B2 and S3 are connected in parallel with an audio frequency source 43. The primary windings S and 55 of the transformers 59 and 5i, respectively, are connected in series and the combination is also placed in parallel with the audio frequency source 43. In this manner, currents are set up in the thermistors 36. The resistances 62 and t3 and the primary windings 64 and 65 form thefour arms of a bridge circuit. Assuming that the two transformers 59 and El are substantially identical, if the resistances of the two thermistors 3S are equal the bridge will be balanced. If they are not equal, the bridgeY will not be balanced and an unbalance voltage will appear across the primary winding 65 of a third transformer el. This primary winding 6% is connected from a point between the two resistances 62 and 53 to a point between the primary windings 64 and 55 of the rst two transformers 59 and El, respectively.

The secondary winding t8 or" the third transformer t? is connected between ground and the iirst grid of a pentagrid converter tube 59. The primary winding E" of a fourth transformer il is connected in parallel with the audio frequency source Gne end of the secondary winding l2 of this fourth transformer il is connected to the positive side of a biasing battery uit, the negative side of which is grounded. Thev other end of the secondary winding i2 is connected to the second grid of the pentagrid converter iid. .a biasing resistor 'i3 is connected between the cathode of the converter El@ and ground. The exact operating biases for the rst two grids oi the converter t9 depend upon the particular t pe oi tube which is used. The biasing battery it and the biasing resistor 'i3 are chosen so as to make the converter E9 operate with both the first and second grids near cut-ofi, i. e., conduct'- ing substantially during only positive cycles of grid current. The plate of the converter GS is connected through a resista-nce 'it to the positive side of a battery i5. The negative side of the battery 'i5 is grounded. A "or denser i5, oiering a low impedance at audio frequencies, is connected in parallel with the series comb ation of the resistance 'ld and the battery u. The plate of the pentagrid converter tube also connectedl to the screen grid of the multigrid tube d, in the heating circuit ci the second thermistor 3B.

The amplitude and phase or" the voltage appearing on the second grid of the converter @S are both xed by the audio frequency source dii. The amplitude of that appearing cn the first grid, on the other hand, is dependent on the amount of unbalance in the bridge. rllhis voltage is of either the same or the opposite phase that on the second grid, depending on the direction of the unbalance. Since the tube 62 is being operated with both grids near cut-cit, the output consists of substantially a positive voltage loop every half cycle. f no imbalance voltage is present on the rst grid, the output will be iixed and have a certain direct component. if an imbalance voltage exists and is in phase with the voltage' from the' source-U63., the" output voltage'loops willbe-largerand the direct coinponent will be greater. 1f the unica-lance voltage is opposite in phase the loops will be smaller and the direct component will be less than that which would exist if there were no unbalance. All audio and higher frequency components of the output wave of the pentagrid converter tube 69 travel through the condenser 18 to ground. The remaining direct voltage is applied to the screen grid of the second multigrid tube 4D. which con'- trols the heating current iowing through the second thermistor 36.

As in the previous example, the general level of the gain of the thermistor-controlled amplifiers is under the control of a rectified pilot voltage. This rectified pilot voltage is applied to the control grids of both multigrid tubes 48. The positive terminal of an additional batteri7 51, the negative terminal of which is grounded, is connected to the screen grid of the first multigrid tube 48. The voltage output of this battery 5i is equal to the direct voltage component of the output of the pentagrid converter tube 89. The thermistor heating current through each multigrid tube 48 is thus affected equally by changes in the amplitude of the pilot signal. If the resistances of the two thermistors 36 are not equal, the bridge will not be balanced and the resulting unbalance signal will either increase or decrease the potential of the screen grid of the second multigrid tube 48, changing the heating current iiowing through the second thermistor 38, and causing a change in resistance in the direction required to remove the unbalance.

Still another embodiment of the present invention is shown in Fig. 4. In this particular example, a circuit for obtaining an output voltage proportional to the product of two input voltages employs two indirectly7 heated thermistors 11 and 18. The resulting voltage multiplication will be exact only if the two thermistors 11 and 'E8 have identical heater current versus resistance characteristics. this embodiment of the invention, however, means is provided to equalize the resistances of the two thermistors 11 and 18, despite any slight dissimiiarity in their heater current versus resistance characteristics, under substantially ali conditions by controlling the relative amounts of heater current associated with each.

Referring now to Fig. 4, the particular example of a voltage multiplier shown employs two high gain direct current amplifiers 19 and 80, the two indirectly heated thermistors 11 and 18, and heaters 8l and 82 for the thermistors 11 and 18, respectively. Self-heating in the thermistors 11 and 18 will be assumed to be negligible. -Direct current sources 83 and 84 furnish slowly varying direct voltages, which are the voltages to be multiplied. The negative side of the first direct current source 83 is connected through a resistance 85 to one terminal of the first thermistor 11 and the positive side is grounded. The other terminal of the first thermistor 11 is connected to the positive side of a fixed biasing battery 86, the negative side of which is grounded and connected to the negative side of the second direct current source 84. The positive side of the second source 84 is connectedto one terminal of the second thermistor 18. The other terminal of the second thermistor 1B is connected to the input side of one of the direct current amplifiers 88. This amplifier 80 is shunted by a resistance v81 which acts as va negative feedback circuit.

The output side of the direct current amplifier 88 is connected through a resistance88 to ground. The input side of the other direct current amplifier 19 is connected to a point between the re- In accordance with l sistor 85 and the first thermistor 11. The out-'- put side of the other amplier 18 is connected to the heater 8l which is associated with the first thermistor 11. The other side of this heater 8l is connected through the other heater 82, which is associated with the second thermistor 18, to ground.

By ordinary action of feedback amplifiers, the output voltage of the direct current amplifier 19 will adjust itself so that the currents flowing to and from the grid of the first stage will cancel each other. In other words, the amplifier 19 output will force enough current through the heater 8i so that the resistance of the first thermistor 11 will alter until the current set up in the resistance 85 by the iirst source 83 cancels that set up in the thermistor 11 by the biasing battery 86. The same current flowing through the first heater 8| ows through the second heater 82. Assuming. for the moment, that the thermistors 11 and 18 have equal resistances for equal heater currents, this makes the resistance of the second thermistor 18 equal to that of the first thermistor 11. Thus, the resistance of the second thermistor 18 is varied under control of the voltage output of the first direct current source 83.

The second direct current source 84 sends current through the second thermistor 18. This current is fed into the direct current amplifier 80. By feedback action of the amplifier 88, as was the case with the other amplifier 19, the output voltage adjusts itself so that the currents owing to and from the grid of the first stage cancel each other and hold the grid at very nearly ground potential. The current flowing back through the shunt feedback resistor 81 will equal that set up in the thermistor 18 by the second source 84. Since the output voltage of the amplifier is the same as that appearing across the feedback resistor 8D, it is proportional to the current in the thermistor 18 or, in other words, directly proportional to the voltage of the second source 84 and inversely proportional to the resistance of the second thermistor 18. Since feedback action of the other amplifier 19 forced the resistance of the first thermistor 11 to adjust until the current set up in it by a fixed biasing battery 86 was equal to that set up in a fixed resistance by the first direct current source 83, the resistance of the first thermistor is inversely proportional to the magnitude of the voltage of the first source 83. Under our assumption, the resistances of the two thermistors 11 and 18 are equal for equal heater currents. Therefore, the output voltage of the second amplifier 80 is directly proportional to both the voltage of the second source 84 and that of the first source 83 or, in other words, their product. This output voltage may be picked off from the output resistance 88.

Since the accuracy of the voltage multiplication is dependent upon the ability of the thermistors 11 and 18 to maintain equal resistances for equal heater currents, it is highly advantageous to employ an embodiment of the present invention to insure such accuracy. In this embodiment of the invention, the secondary winding 89 of a transformer 98 is connected in series with the first thermistor 11 and the biasing battery 88 of the voltage multiplier circuit. The secondary Winding 9| of another transformer 82 is connected in series with the second voltage source 84 and the second thermistor 18. The primary windings 83 and 94 of the transformers 90 and 92, respectively, are then connected in series and 1l forni two arms of a bridge circuit. Two substantiall'yl equal resistances 95 and 95 are con-` nected'in series and, when the combination is connected in parallel with the series connected nrimary transformerY windings 93 and 9e, form the othertwo arms of the bridge.

An alternating current source 91 is connected between the pair of opposite bridge terminals formed'by thejunctions of the resistances 95 and 96 and of the Vprimary transformer windings S3 and 94. The'terminal formed by the junction of the primary windings 93. and 94 is grounded. The terminal formed' bythe junction of the resistances. 95 and 95 is` v connected through another resistance 98 to one side 'of an additional heater 99' for the second thermistor '18, the other side of the heaterSS being grounded. rljhe two remaining bridge terminals are connected to the input terminals of 'a' high gain amplier itil.

One'output terminal 'of the high gain amplier I lii,` is grounded and the other is connected thrOllgh a resistance to the, Ungrounded side of the additional heater e9. rIjhe regular heater 82 for thev second thermistor 'i8 is shunted by a resistance |92 for the purpose of reducing the heating effect of the regular heater 82 to accommo'date that of the additional heater 99e.

Several additions are' necessary in' thevoltage multiplier circuit to prevent the alternating.cur"-Y rent which has been introduced from interfering withl its operation. A condenser Ii is connected between ground and a point between the rst thermistor 'ilv and the resistance 8,5, blocking direct currentv but forming a 'path for alternating current. An inductance Illis connected between the first direct current amplier i9 and the heaterzl to block alternating current from theV heater circuit. An additional inductance |85 is connected between theV second direct'current amplifier 89 and the output resistance 83 to block alternating current from the output circuit.

A biasing voltage is applied tojthe additional heater 9 9 trom the alternating current source Si? through the' isolating resistance 98. The v alueoi" the resistor 102. shuntin'g the Yregular heater 820i thesecond thermi'stojr 1 8; is chosen so as` to make the'l total heating effect' of 'the' Vtwo lfieaters'` 82' and 99 'Equal '00 -Ithey original heating effect oi the' regularl heaterv 82 alone. If the resistances vof the two' vthelrnistors 77|' and 'I9 are equal, the' bridgewill be'balan'ced and no correction` Will be made. However, Vii 'thereSiStances 'of the two thermistors 'il and' 'i8 are' not equal, the bridgek will,V not be balanced andan unbalancejvolt'age willeppeargacross theiinput terminals of the high gain amplifier |00, the'aiplitude of the unbalance voltage depending on ithe 'amount of unbalance andthe phasedependingon its direction, The output of the amplifierv |091 isifed through thejothe'r isolatingresistor` |0| to the additional heaterV` 9,9. vThe]isolatingresistors 93 and lol",

forcethe'l currentfsthrough 'themto add vectorial'- ly andenterthe heaterf." Thejresultant voltagejwhichj isA applied tothe.' materv 99' is greater than the bias if b hGu'nbalance. voltage is in phase withr the alternatingV current source voltagev and isles/s than theV biasif the imbalance voltage is opposite in phas'e. This resultant signal tends to either raise orl lm'gerl the resistanceV of the second thermistor l, removing theY unbalance and 1 1 "and '187.1v

The 'speciilc embodiments of they invention which have' been described are illustrative of the equaiizing the resistancesfof ythe two thermistors applications which ere'possiblein. these se@ other embodiments, "diierent combinations or frequencies,` different circuitv configurations, and different circuit elements may be used to advan-, tage. Many other embodiments of the general principles involved in the present invention may be made within the spirit and scope of the an#v pended claims.

What is claimed is:

l. In combination, two currentfcontrolled variable resistances having substantially similar resistance Versus vcontrol-current characteristics. a source of current, connections from said source to each of said variable resistances, means to compare the currents owing from said source to each of said variable resistances, and means responsive to said comparison means or maintaining substantial erguiality4 between the resistances of said variable resistances.

2. A combination'i'n accordance with claim 1 in which said last-mentioned means includes means to change the control current of only one 0f said variable resistances in any one direction.

3. A combination in accordance with claim l inwhich said last-mentioned means includes 4means to change the control current of only one of said variable rsistances.

4. Acombinationin accordance with claim 1 in which' said"last`rnentioned means includes means to increase 'the control current of one of said variable Vresistanc'es. and means to decrease the control current ofthe'otherof. said variable resistances.

5., In combinationi two current-controlled variable resistances having substantially similar.. resistance versus control-current characteris-A tics, a source of voltage, two transiormers, connections from one winding vof one of said transformers to one of said variable resistances, connections from one winding of the otherof said transformers tothe other of said variable re# sistances, connections.v from said source to each of the remaining'windingg ofY said transformers, means to compare the currentsv set up by said1 source in each of said variableresistances andA means responsiveto saidl comparison meansfor maintaining substantial equality between the rel sistances ofsaid variable resistances.A 'A

6- In. Combination'. two. 'indirectly heated; ihermisiors having substantially' Similar resist: ance versus ltemperature characteristics, enter-7. nalfelectrical: heatingunits for each of said ele-y ments, a voltage source, connections from said source to said heatingunits, an additional hearting unit for one of.- saidthermistors, an addi-` tional voltage` sourcel connections fromsafid additional sourceV toY each. ofv said thermistors means. to compare, thecurrents set up by said additional source in,r each ofv said: thermistors,v and means responsive to, said comparison-,means for maintaining substantialA equality betweenv the resistances, ofsaid-thermi'stors by adjusting the ,temperature of said-V additional` heating unit-.

7. In combinatiom two indirectly; heated; thermistors having, substantially similar resist- V ance versus temperature, characteristics, means forl detlcting anydiiierence between theirresiste, ances ofsaid thermistors comprising ai( voltage source, twotransforrners, and. two's'ubs'tantially equal resistances,V thejpriinary .windingsfofsaid. transformers said. substantially .equal resistff ances forming I a bridge', circuit to "compare 1 they currents setupA by i said source.1 in each f of. said` therrnstors, the secondary 'windings of said transformers being connected to said thermistors,

aces-,eos

and said source being connected between two opposite terminals of said bridge, an additional external heating unit for one of said thermistors, and means responsive to any unbalance in said bridge connected to said additional external heating unit for modifying the resistance of its associated thermistor so as to balance said bridge and remove any difference between the resistances of said thermistors.

8. A combination, according to claim 7, in which said means responsive to any bridge unbalance comprises a high gain amplifier, connections from the remaining pair of terminals of said bridge to the input side of said amplifier, connections from the output side of said amplier to said additional heating unit, and connections from said voltage source to said additional heating unit.

9. In combination, a pair of thermistors having substantially similar resista-nce versus temperature characteristics, a source of voltage, circuit means for applying voltage from said source to each of said thermistors and thereby establishing currents in each of said thermistors, means for comparing said currents comprising a bridge circuit that is balanced when said currents maintain substantial equality, means for detecting any bridge unbalance, and means responsive to any unbalance detected by said detecting means for altering the resistance of at least one of said thermistors in such direction as to cause said currents to regain substantial equality, thereby causing said bridge to regain said balance.

10. A combination in accordance with claim 9 in which said last-mentioned means includes means to change the temperature of only one of said thermistors in any one direction.

1l. A combination in accordance with claim 9 in which said last-mentioned means includes means to change the temperature of only one of said thermistors.

12. A combination in accordance with claim 9 in which said last-mentioned means includes means to increase the temperature of one of said thermistors and means to decrease the temperature of the other of said thermistors.

13. In combination, two current-controlled variable resistances having substantially similar resistance versus control-current characteristics, mean for comparing the resistances of said variable resistances comprising two transformers, at least one of said transformers having a divided secondary winding, a voltage source, and

connections from said source to the primary winding of one of said transformers having a divided secondary winding, said means forming a bridge circuit, two adjacent arms of said bridge circuit being the portions of said divided secondary winding, the other two adjacent arms being said variable resistances, and the primary winding of the other of said transformers being connected from a point between the portions of said secondary winding to a point between said variable resistances, and means responsive to said comparison means connected to the secondary winding of the other of said transformers for maintaining a predetermined relation between the resistances of said variable resistances.

14. In combination, two current-controlled variable resistances having substantially similar resistance versus control-current characteristics, two electrical discharge devices, a source of direct current, connections to carry current from said source through one of said discharge devices to one of said variable resistances, connections to carry current from said source through the other of said discharge devices to the other of said variable resistances, a source of alternating current, connections from said alternating current source to each of said variable resistances, means to compare the currents set up in said variable resistances by said alternating current source, and means responsive to said comparison means for maintaining a predetermined relation between the resistances of said variable resistances by modifying the flow of direct current through at least one of said discharge devices.

i5. A combination in accordance with claim 14 in which said last-mentioned means comprises means for maintaining equality between the resistances of said variable resistances.

16. A combination in accordance with claim 14 in which said last-mentioned means includes means to change the control current of only one of said variable resistances.

17. A combination in accordance with claim 14 in which said last-mentioned means includes means to increase the control current of one of said variable resistances and means to decrease the control current of the other of said variable resistances.

18. In combination, two amplifier circuits, two current-controlled variable resistances having substantially similar resistance versus controlcurrent characteristics, one of said variable resistances being connected in a gain-controlling relation to each of said amplifier circuits, asource of current, connections from said source to each of said variable resistances, means to compare the currents owing from said source to each of said variable resistances, and means responsive to said comparison means for maintaining a predetermined relation between the resistances of said variable resistances, thereby maintaining a predetermined relation between the gains of said amplifier circuits.

19. In a system containing two distinct ampli- Iier circuits and two current-controlled variable resistances having substantially similar resistance versus control-current characteristics, each of said amplifier circuits having one of said variable resistances connected in a gain-controlling relation, a source of variable control voltage, means for varying the general level of the gains of said amplifier circuits under the control of variations in said control voltage by varying the resistances of said variable resistances, a source of calibrating voltage, connections from said calibrating voltage source to each of said variable resistances, means to compare the currents flowing from said Calibrating source to each of said variable resistances, and means responsive to said comparison means for maintaining a predetermined relation between the resistances of said variable resistances, thereby maintaining a Dredetermined relation between the gains of said amplifiers.

20. In combination, a signal transmission line, a pair of amplifier circuits connected in parallel with each other incorporated in said transmission line, a pair of self-heated thermistors having substantially similar resistance versus temperature characteristics, one of said thermistors being connected in a gain-controlling relation in each of said amplifier circuits, a source of pilot voltage operating at a frequency different from the signal frequency of the line, connections from said pilot voltage source to said transmis- 'acari-,coc

15 sion line to enable the'pilot voltage to be transmitted over said line, at 'least one circuit for iiltering the pilot voltage from the main signal, connections from said line to said iter circuit, means for rectifying said pilot voltage, connections from said lter circuit to said rectiiying means, connections from said rectifying means to each of said thermistors, thereby enabling the general level of resistance of said thermistors to be varied under the control or variations of said pilot voltage, an additional alternating 'current source operating at a frequency diiierent from the main signa-l frequency, connections from said additional source to each or said Vther- -mistors, means for comparing the currents iiowing from said additional source in said thermistors, and means responsive to said comparison means for maintaining a predetermined relation between the resi-stances of said thermistors, thereby maintaining a predetermined relation between the gains of said amplifier circuits.

21. In combination, two amplifier circuits, each of said amplier circuits transmitting an alternating current signal, two vcurrent-controlled variable resistances having substantially similar resistance versus temperature characteristics, one of said variable resistances being conu nected in a gain-controlling signal-transmitting relation in each of said amplifier circuits, an

alternating current source operating at a frequency dii-ferent from the main signal frequency, connections from `said source to each of said variable resistances, means to confine said signal to said amplier circuits, means to corinne current from said source to said variable resistances and said connections from said source, means to compare the currents in each of said variable resistances set up by said source, and means responsive to said comparison means for maintaining a predetermined relation 'between the resistances of said variable resistances, thereb y maintaining a predetermined relation between the vgains 0f said amplifier circuits.

22. In combination,a signal translating circuit includinga pair of impedance elements having Asubstantially similar high temperature coefcients of resistance, each of said .elements corinected to be traversed by translated signals, a pair of circuits each including a respective one of said impedance elements and a common voltage source for passing through each of said elements a current of a strength insuiiicient to substantially aiect the resistance thereof and which is dependent upon the resistance of said element, circuit means, including a source of heating current, for altering the temperature of one of said elements relative to the vtemperature of the other, means for comparing said currents established by said common voltage source in said elements, and means including said circuit means and actuated by-said last-mentioned comparison means for maintaining a Vpredetermined relation between said compared currents, vwhereby apredetermined relation between the respective resistances of said elements is accurately maintained.

23. A combination in accordance with claim 22 in which the frequency of the translated signals and the frequency of said comparison currents are different, and with frequency vselective means connected into said combination to exclude the signal from the'comparison A circuit.

EMRY LAKATOS. BROCKWAY MCMILLAN.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 2,104,336 Tuttle Jan. ,4, 1938 2,126,529 Black Aug. 9, 1938 2,206,080 Davis July 2, 1940 2,209,667 Taylor July 30, 1940 2,278,633 Bagnall Apr. 7, 1942 2,412,227 Och et al Dec. 10, 1946 FOREIGN PATENTS Number Country Date 366,920 Great Britain Feb. 8, 1932 

